The present invention relates to a fastener driving tool such as a nail gun driven by compressed air, and more particularly, to such fastener driving tool improving drive response and decreasing air consumption.
Heretofore, fastener driving tools such as nail guns have existed which drive fasteners such as nails or staples using compressed air as the power source. In such fastener driving tools, compressed air is supplied to a piston upper chamber defined by an inner surface of a cylinder and a piston for rapidly displacing the piston to perform nailing. Compressed air is supplied from an external source and temporarily stored in an accumulator formed within a frame of the nail gun. The accumulator and the piston upper chamber are connected by a channel, but one or more valves which are switched between open and shut-off positions are provided along this channel. These valves are designed to open or shut-off the channel by supplying or expelling compressed air in valve chambers constituted by the spaces each adjacent to each valve. Typically the structure is such that a first valve is activated as a result of external operation of a trigger or the like, and this operation allows a downstream passage to be communicated with or to be shut-off from the first valve. Thus, a downstream valve chamber is brought into communication with or shutting-off from the upstream passage, thereby sequentially activating or deactivating the downstream valves.
In addition, a time period starting from completion of the nail driving operation to restoration to an initial state for the next nailing operation is dependent upon the circulation speed of the compressed air in the fastener driving tool after the trigger is released, and the movement speed of the valves in proportion to this circulation speed. That is, the time period is dependent on the shut-off speed for shutting off the piston upper chamber in the cylinder from the accumulator by a valve caused by, after releasing the trigger or the like, circulation of the compressed air through the channel in the fastener driving tool as a result of the returning motion of a plunger which had been pressed by this trigger.
In a conventional fastener driving tools as disclosed in Japanese Patent Publication No. S58-50833, valve activation is performed sequentially from valves whose valve chamber volume is small to valves with large valve chamber in order to stabilize operation of the valves irrespective of the speed with which the trigger is pulled. Since with this structure the valves are sequentially activated by compressed air, a time period starting from pulling the trigger and/or pushing operation of a push lever against a workpiece to a start of the nailing driving motion is highly dependent upon the time required to sequentially activate the valves.
In order to reduce this time period and increase response, Japanese Patent Publication No. H7-112674 discloses a nail gun, in which a main valve is divided into first and second valves, so that kinetic energy of the first valve is utilized to improve the operating speed of the second valve.
With this structure in which the main valve is divided into two valves, only the time period from when the second valve begins to move until it moves to maximum displacement is reduced. The time period from both pulling the trigger and pushing the push lever onto the workpiece to the operation timing of the first valve is still not reduced. In addition, since only the time period from when the second valve begins to move until it moves to maximum displacement is reduced, it was only possible to reduce the time period from when the trigger is pulled until nailing is performed. Consequently, a time period from the completion timing of the nail driving operation to the start timing of the next nail driving operation cannot be reduced when continuous nailing is performed. That is, a response cannot be improved.
Laid-open Japanese Patent Application Kokai No. H11-33930 discloses a structure in which, an internal volume of a main valve chamber for accommodating therein a main valve is increased. With this arrangement, air damping behavior due to compression of the main valve chamber does not occur when the main valve rises and is contained in the main valve chamber.
With this structure in which the volume of the main valve chamber is increased, the amount of compressed air accumulated in the main valve chamber increases. For this reason, the time period for discharging the compressed air out of the main valve chamber is increased, which degrades the response.
Laid-open Japanese Patent Application Kokai No. H5-138548 discloses communication of a piston lower chamber with a trigger valve chamber. The movement speed of a valve piston and a main valve are increased as a result of the pressure which is generated from the movement of the piston.
With this structure in which the piston lower chamber and trigger valve chamber are connected, at the instant that the piston passes through the one-way valve disposed at an intermediate region of the cylinder, compressed air flows into the trigger valve chamber and closes the main valve. Therefore, the nailing force was reduced. Moreover, extremely complicated structure results.
Another conventional fastener driving tool has been proposed. The tool includes a trigger valve and main valve. A trigger valve exterior frame internally defines a trigger valve chamber. The trigger valve includes a plunger extending through the trigger valve exterior frame and the trigger valve chamber and slidably movable as a result of the movement of the trigger and the abutment of the push lever against the workpiece. The movement of the plunger selectively shuts off a fluid communication between the accumulator and the trigger valve chamber and between the trigger valve chamber and an atmosphere. However, the resultant arrangement cannot provide high response for discharging compressed air from the main valve.
Still another conventional fastener driving tool is proposed in which a main valve is not provided, but a trigger valve is additionally equipped with a valve piston. The valve piston is reciprocably slidably disposed in a trigger valve exterior frame, and has one side in the sliding direction facing the accumulator. The valve piston alternately opens and blocks a channel from the piston upper chamber connected to the trigger valve exterior frame to the accumulator and a channel from the piston upper chamber to the atmosphere. With this fastener driving tool, the displacement of the valve piston serves to select the air channel and control the nailing of the fastener. However, the speed of the displacement of the valve piston is low, and the delay in the displacement of this valve piston can cause other control to be delayed as well. Consequently, the problem arises that the time lag from when the operator begins the nailing operation until the fastener is actually driven becomes large, response becomes poor to lower workability. In addition, the problem arises that when many fasteners are to be driven in a short period of time, the aforementioned time lag makes continuous nailing difficult to perform.
In addition, with the conventional fastener driving tools, after nailing, in order to return the piston to the pre-nailing position, the piston upper chamber and the atmosphere are communicated with each other for releasing the compressed to the atmosphere, while the valve is closed for preventing the compressed air from flowing from the accumulator into the piston upper chamber.
However, during the period from when the valve begins to close until it is completely closed, the accumulator and the piston upper chamber are communicated with each other, and the piston upper chamber and the atmosphere are also communicated with each other. Accordingly, the compressed air in the accumulator would in some cases flow unnecessarily into the piston upper chamber and is expelled into the atmosphere. This causes an increase in air consumption, which consequently requires a high-performance compressor or the like to produce compressed air.